O-hydroxy Schiff bases

Zhuo investigated O chemical shifts of o-hydroxy Schiff bases. These systems are in some instances tantomeric. As described previonsly O chemical shifts are very good indicators of tantomerism (see Section n.K.l). Provided that good reference values for the two tautomeric states exist, the equilibrium constant can be determined. Zhuo used the values for simple Schiff bases as models for the phenolic form (48). For the form 48B a value from a simple enamine was chosen. This, however, is not a very appropriate choice, as it does not at all take into account the charged resonance form (48C). The equilibrium constant determined for Af-(2-hydroxy-l-naphthalenyhnethylene) amine is quite different from that derived by /(N,H) coupling constants. ... 

Figure 6.26 Tautomeric equilibrium of o-hydroxy Schiff bases.

The approach will be an exemplary one presenting selected cases rather than to review the literature. Many fine examples have been provided already in Ref. [1]. The molecules to be used are o-hydroxy Schiff bases, 1,2,3-triazole, o-hydroxy azo... 

Tautomerism of o-Hydroxy Schiff bases can be studied using chemical shifts... 

Figure 3.14 Plot of deuterium isotope effects on C chemical shifts for o-hydroxy Schiff bases. (Reproduced from Filarowski et al. [24]. Copyright (2005), with permission of American Chemical Society.)...

These have been studied in o-hydroxy Schiff bases (Figure 3.7) as seen in Figure 3.17. Again, a clear S-shaped curve Is seen and tautomerism can be easily established [26]. 

One-bond couplings in systems like o-hydroxy Schiff bases (see Figure 3.7) are a possible way of detecting tautomerism. A prerequisite is again to know the values for the two forms. These have been estimated from o-hydroxy Schiffbases (Figure 3.7) with no substituents at the benzene ring (OH-form) and from, for... 

Cases of both UV/VIS and NMR spectra of o-hydroxy Schiff bases are known [23]. The UV spectrum is shown in Figure 3.29. However, to estimate the equilibrium constant the extinction coefficient for both species need to be known. 

Deuterium isotope effects on chemical shifts have been recorded for o-hydroxy Schiff bases (Figure 3.7). One example of the use of is Schiff bases of 2-hydroxy-l-naphthaldehydes [49]. Based on intrinsic effects of-1.6 ppm for... 

An example of the use of theoretical calculations in a tautomeric system is shown for o-hydroxy Schiff bases in Table 3.2. 

The reaction of Zr(OPr ) with the Schiff bases acetylacetone-2-hydroxy-alkylimine, N-[(3-hydroxy-2-naphthyl)methylene]-2-hydroxyalkylamine, and o-hydroxyacetophenone-2-hydroxyalkylimine (HL) affords the polymeric Zr(OPr )2L and ZrL2 compounds. N.m.r. and i.r. spectral studies indicated that the N atom of the Schiff base is co-ordinated in these compounds. 

ZT-1 is a Schiff base derivative from natural HA, and its chemical name is [51 -(5a,9p,ll )]-5- [(5-chloro-2-hydroxy-3-methoxyphenyl)methylene]amino -ll-ethylidene-5,6,9,10-tetrahydro-7-methyl-5,9-methanocycloocta[b]pyridin-2(lH)-one. ZT-1 is a prodrug and is transformed nonenzymatically into the active compound HA. In aqueous solution, ZT-1 is rapidly degraded into HA and 5-Cl-o-vanillin by hydrolysis. Now, ZT-1 is being developed as a drug candidate for the treatment of AD by Debiopharm S.A. of Switzerland. 

The reduction by Na(Hg) of several o-hydroxynaphthalenecarboxylic acids in slightly acid solution containing boric acid has been investigated. 1-Hydroxynaphthalene-2-carboxylic acid [109] is, like salicyclic acid, reduced to the aldehyde (62% yield), isolated as the Schiff base, whereas 2-hydroxynaphthalene-3-carboxylic acid [110] under similar conditions yields mainly 2-hydroxy-1,2,3,4-tetrahydronaphtha-lene-3-carboxylic acid (75-80%) and 5% of the aldehyde. When hydrochloric acid rather than CO2 was used to neutralize the base formed during the reduction, the 2-oxo-l,2,3,4-tetrahydronaphthalene-3-carboxylic acid [111] was isolated (60%). On heating, the compound, a y6-keto acid, decarboxylated. 

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